The present invention relates to the removal of acetylene present in an hydrogen chloride stream by reacting said stream with oxygen over an oxidation catalyst, thereby converting said acetylene to non-chlorinated hydrocarbon products, i.e., carbon monoxide and/or carbon dioxide.
It is commonly known in the art that 1,2-dichloroethane can be cracked at elevated temperatures to produce vinyl chloride monomer with hydrogen chloride being formed as a by-product. This HCl by-product is commonly recycled and used in the oxychlorination of ethylene to form 1,2-dichloroethane. However, in the thermal cracking step, small amounts of acetylene, e.g., typically 2000-3000 ppm, are formed and subsequently converted, in the oxychlorination step, to trichloroethane, tetrachloroethane and trichloroethylene. These materials are detrimental to the production of 1,2-dichloroethane as they are difficult to remove and, in the cracking step, form derivatives which inhibit the polymerization of the vinyl chloride monomer product.
While the concentration of acetylene in the HCl streams noted above typically is on the order of about 3000 ppm, this concentration is too low to permit economic recovery of the same or its derivatives in the 1,2-dichloroethane. Further, it is impractical to separate the acetylene from the HCl as the boiling points thereof are close together. In other processes, chlorination of the acetylene has been utilized, but the removal of the chlorinated reaction products from the HCl is also difficult and expensive. Adsorbtion of acetylene upon a solid adsorbent is effective, but the adsorbent must be periodically subjected to purging by an inert gas or liquid, necessitating the construction of two or more adsorption units and thereby entailing a high capital cost. Another difficulty with this type of process is the need to dispose of the gas or liquid stream contaminated with chlorocarbons formed during the purging step. Catalytic hydrogenation of acetylene to ethylene and ethane is also known; however, the cost and hazard associated with the handling of hydrogen are disadvantageous. Likewise, the high energy cost associated with the non-catalytic oxidation of acetylene at high temperatures of 400-500.degree. is a disadvantage.
Accordingly, an object of the present invention is to provide a process for economically removing or reducing the concentration of acetylene in an HCl stream.
Another object of the present invention is to provide a process for improving the purity of the 1,2-dichloroethane produced in the oxychlorination of ethylene. A further object is to provide a process whereby the production of vinyl chloride or other chlorinated hydrocarbons from acetylene in the HCl stream is minimized or avoided and little or no vinyl chloride is formed. Finally, an important object of the present invention is to provide a process which requires temperatures no higher than those typically used in the oxychlorination of ethylene reaction, thus realizing energy savings over non-catalytic oxidation processes.